His exemptions from the definition ranged from teenage boys to men in their 30s, which differed from his later interview with the BBC in which he said the upper limit was 20.

He said: "We are talking about very young people [when] it is paedophilia, and I wouldn't want to mix up kids who are just the wrong side of 16 with paedophiles.

"I don't actually personally adhere to the 15-year-old being with a 20-year-old boyfriend being paedophilia, or even if the boyfriend is 30."

Mr Grange, who stressed his views were personal, said: "You take a look at the circumstances and try to make the right decision for that case.

"It may be nothing, it may be formal warnings, it may be prosecution."

'Pretty normal'

But, speaking to the BBC on Sunday morning, Mr Grange defined the grey area as where "the girl is 13, 14, 15 and the boy is 16, 17, 18, 19, possibly 20".

He added: "If you prosecute each and every time a boy has sex with a girl under 16 and above 12, then we'd be in the schools across Britain, and in the youth clubs across Britain pretty regularly because, since I've been alive, it's been pretty normal.

"It's what teenagers do."

But he said sex between a man over 20 and a girl under 13 was clearly rape and UK law reflected this.

"If the fella concerned - usually a man - is in his 20s, 30s, 40s and 50s, then I believe you should prosecute them ruthlessly, and the law allows for that," he said.

'Dicey ground'

Official government guidance on prosecuting suggests that teenagers who have sex when one of them is below 16 should not necessarily be prosecuted.

Girls, nine, 10 and 11 are now post-pubescent. They're wearing bras, they're having their periods. They are no longer children in his definition

Kidscape

Michelle Elliott, the director of charity Kidscape, which campaigns to keep children safe from harm or abuse, told BBC Five Live that using puberty as a guide to sexual maturity was difficult.

"Girls, nine, 10 and 11 are now post-pubescent. They're wearing bras, they're having their periods. They are no longer children in his definition.

"So I think he's on very dicey ground here. I think he was probably trying to bring up the teenage issue, and maybe got himself in deeper than he meant."

Lib Dem MP for Brecon and Radnorshire Roger Williams accused Mr Grange of causing "concern and confusion" with his comments.

"The law is quite clear that having sex with somebody under 16 is an offence," he said.

"We are trying to make it clear that young people are very vulnerable, especially to exploitation by older people, and it just isn't helpful at all."